Description

The Presidential flag of Singapore is red with a white crescent and five stars, all pointing
to the fly. Source: Barraclough and Crampton 1981.
Željko Heimer, 5 July 1996

From having seen the Singapore President flag first-hand, I believe that its proportion is 2:3 similar to that of the Singapore flag as I have seen both of them side by side and it has never struck me that one was of different proportion to the other. I have an actual photo of the Singapore President flag flying from the Istana (palace) and will scan it for anyone interested.Tom Koh, 17 January 2000

Barraclough and Crampton 1981 is unclear: it says the presidential flag is made of a canton of the national flag. The national flag is 2:3, and therefore
they might have intended to imply that the presidential flag also is. Also, we have Tom Koh's
first-hand testimony and picture, plus the 2:3 ratio in the
–elsewhere not very reliable– Dorling-Kindersley 1997.
However Smith 1975, Smith 1985 and Album des Pavillons 2000 all show the presidential flag in a 1:2 ratio, all being generally reliable sources. It just occurred to me, particularly considering that Album des Pavillons 2000 is a naval reference, that maybe both ratios are correct  the 2:3 one being used on land, the 1:2 one at sea?
Santiago Dotor, 11 April 2002

The website of the Office of the President of the Republic of Singapore, better known as the Istana, has a protocol webpage with the description and symbolism of the Presidential Standard and also features the Presidential Arms [image here]:

The President's Personal Standard is a red flag with a white crescent moon and five stars emblazoned in the centre. The colour red is symbolic of the universal brotherhood and equality of man. The crescent represents a young country on the ascent while the five stars represent democracy, peace, progress, justice and equality.

The President's Personal Standard is flown from the highest point on the Istana main building. The Standard is flown daily from 8.00 am to 6.00 pm, or until such time when the President has left for his private residence.

Herman Felani M.Y., 21 November 2002

The flag illustrated in that Istana webpage confirms the 2:3 ratio, however the crescent-and-stars motif appears much smaller than in the above image.
Santiago Dotor, 22 November 2002